Casting practices that dictate grain size and microstructure have significant affect on material properties such as strength and ductility. These same practices may also affect consistency and quality of the castings. Castings employed in gas turbines, for example, are either equiaxed, directionally solidified, or single crystal in nature. Many of these cast components require additional fabrication steps to achieve the final product.
Techniques associated with fine grain control during casting include, e.g., agitation to break up dendrites as they form, increase nucleation sites and inhibit grain growth; inoculation with compounds of lower melting point to increase nucleation sites and inhibit grain growth; and maintenance of low pouring temperature to promote fast freeing and inhibit grain growth. Unfortunately these techniques have particular drawbacks. For example, agitation can introduce microporosity and require post cast hot isostatic pressing to close pores. Inoculation can introduce non-metallic inclusions which can initiate fatigue cracks.
Additionally, and similarly to the above practices, maintaining low pouring temperature is also complex and relatively inflexible. For example, techniques associated with large directional or single crystal grain control may include very slow extractions, e.g., a few inches per hour, of the solidified part from the hot zone of a Bridgman vacuum furnace; also, pre-coating the alumina mold with a nucleation inhibitor to prevent lateral grain growth. Due to the complexity and inflexibility of the aforementioned practices, a need remains to improve management of grain growth during solidification.